1. Field of the Invention
The invention relates to a method of encapsulating materials by entrapment in a matrix of water-insoluble polyhydroxy polymers and to the compositions prepared thereby.
2. Description of the Prior Art
Prior art methods of encapsulation can be described in two major categories, physicomechanical and chemical. Physicomechanical techniques include the following:
a. Spray drying: An emulsion is prepared with a film-forming polymer dissolved in the continuous phase. The emulsion is then dried by spraying into a stream of hot inert gas. Before spray drying the wall materials can be strengthened by crosslinking the polymer wall material. See U.S. Pat. Nos. 3,016,308 and 3,429,827. PA1 b. Dipping or centrifuging technique: Core material particles or droplets are passed through a thin film of liquid wall-forming material. The wall material is then hardened. See U.S. Pat. No. 3,015,128. PA1 c. Multiple nozzle spraying: Core material is sprayed from an inner orifice while the wall material is sprayed from a concentric ring orifice. In this manner water or aqueous solutions are encapsulated in paraffin or other waxes. See U.S. Pat. No. 3,423,489. PA1 d. Fluidized bed coating: Particles are held suspended by a vertical stream of air and sprayed with wall material which, after evaporation of solvent, forms a solid film around the core material. This technique is used when solid particles are to be encapsulated. PA1 e. Electrostatic microencapsulation: Atomized core material and molten wall material are oppositely charged and mixed in a collision chamber. The thusly encapsulated particles are held in suspension and cooled to form the powdered product. See U.S. Pat. No. 3,159,874. PA1 f. Vacuum encapsulation: Wall material is volatilized in a vacuum and deposited on colder nonvolatile core material particles which are in a rotary motion. PA1 a. Coacervation: The attraction between colloids and water of solvation is altered to such an extent that the colloid particles will tend to aggregate to form two separate and distinct liquid phases within the colloidal suspension. Both phases contain the same components with one phase (the coacervate) having a much greater concentration of colloid than the other. PA1 b. Interfacial polymerization: This method necessitates the use of at least a two-phase system. One of the reactants must be soluble in the continuous phase and insoluble in the discontinuous phase (core material). The other reactant must be insoluble in the continuous phase and soluble in the discontinuous phase. The polymerization reaction occurs at the interface between the two phases forming a polymer shell around the core material, thereby completely enveloping it. This shell must be insoluble in both phases. In this method either phase can be an aqueous system. See U.S. Pat. Nos. 3,577,515 and 3,575,882 and British Pat. No. 1,163,023. PA1 a. preparing a dispersion or solution of a suitable chemical biological agent in a first matrix-forming material comprising an aqueous solution of a polyhydroxy polymer xanthate (PPX) having a xanthate degree of substitution of from about 0.1 to 3, wherein the relative amount of said PPX with respect to said biological agent is sufficient to entrap said agent within a matrix of said PPX; PA1 b. reacting the PPX with a coupling agent at a pH of from about 2 to about 7 to form a first insolubilized matrix thereby entrapping said agent; PA1 b'. optionally redispersing said first matrix from step (b) in a second matrix-forming material comprising an aqueous solution of PPX; PA1 b". reacting the PPX in step (b') with a coupling agent at a pH of from about 2 to about 7 to form a second insolubilized matrix thereby further entrapping said agent; and PA1 c. recovering said entrapped chemical biological agent.
The most important chemical encapsulation techniques include the following:
The encapsulation occurs when small droplets of oil (a completely water-immiscible liquid) are present in the colloidal suspension. As the coacervate is formed it is deposited around individual droplets. The coacervate must then be hardened (gelled) by lowering the temperature below the gel point. The capsules are then dehydrated and permanently hardened.
(1) Simple coacervation: A single colloid is dispersed in water and the water of solvation is removed from around the colloid by addition of chemical compounds which have a greater affinity for water than the colloid (e.g., salts or alcohols). This causes the colloid chains to come closer together and form the coacervate. PA3 (2) Complex coacervation: Ionic charges on the colloid chains are neutralized by mixing two colloids carrying opposite charges. See U.S. Pat. Nos. 2,800,458 and 2,800,457.
For additional information and references see "Microencapsulation, Processes and Application", J. E. Vandegaer, ed., Plenum Press, New York and London, 1974, pp. 1-37 and 89-94; W. Sliwka, Agnew. Chem., Internat. Edit., Vol. 14, No. 8, pp. 539-550, 1975; and "Capsule Technology and Microencapsulation", M. Gutcho, ed., Noyes Data Corporation, Park Ridge, N.J., 1972.
The above encapsulation methods are multistep processes which require carefully controlled conditions or special equipment. They are time consuming and expensive, often requiring elevated temperatures and pressures other than ambient; and they all require at least a two-phase system. Many require expensive, toxic, and flammable solvents which must be recovered. Coacervation is limited to the encapsulation of oils in materials which have the capacity to form gels. Interfacial polymerization techniques, also requiring two or more phases, are limited essentially to expensive synthetic polymerization systems, many of which are petrochemicals and which generally produce nonbiodegradable polymers. To make these systems more economical and to prevent ecological contamination, unreacted monomers must be recovered. The only system that appears to be useful for coating solid particles is the fluidized bed technique.